In the lithographic process, a photoresist is applied as a thin film to a substrate (for example, SiO.sub.2 on Si), and subsequently exposed through a mask or reticule. The mask contains clear and opaque features that define the pattern which is to be created in the photoresist layer. Areas in the photoresist exposed to light transmitted through the mask are made either soluble or insoluble in a specific type of solution known as a developer. In the case when the exposed regions are soluble, a positive image of the mask is produced in the resist (a positive resist). On the other hand, if the unexposed areas are dissolved by the developer, a negative image results (a negative resist). After developing, the areas no longer covered by resist are removed by etching, thereby replicating the mask pattern in that oxide layer.
Conventional photoresists are three-component materials, consisting of: the resin, which serves as a binder and establishes the mechanical properties of the film; the sensitizer (also called the inhibitor), which is a photoactive compound (PAC); and the solvent (which is different than the developer solution), which keeps the resin in the liquid state until it is applied to the substrate being processed. The sensitizer is the component of the resist material that reacts in response to the actinic radiation (the property of radiant energy by which photochemical changes are produced). The sensitizer gives the resist its developer resistance and radiation absorption properties.
The solvent is usually inert to the incident imaging radiation. That is, it does not undergo chemical change upon irradiation. The solvent and resin combination, however, directly affects the resist film's adhesion and etch resistance characteristics. The combination also determines other film properties of the resist such as thickness, flexibility and thermal flow stability. As a result, the choice of solvent has a direct impact upon process latitudes--the ability to produce critical dimensions within the specification limits in the face of the process variations encountered during production. For example, in order to get the photoresist to disperse properly across the semiconductor wafer the percentage of solvent-to-photoresist volume must be maintained above a certain minimum level. Conventional wafer processes use ethyl lactate (EL) or propyleneglycol monomethylether acetate (PGMEA) as the solvent material. Both of these materials have a relatively high evaporation rate, which accelerates the drying process. To compensate for the high evaporation rate and allow the photoresist to sufficiently disperse, the amount of resist used per wafer must be increased. These conventional processes on average require about 4.5 cc of resist per wafer. When one considers the volume of wafers processed over, for example, a years time, this is a significant amount of resist and a significant part of the cost of processing. What is needed is a way to reduce the amount of resist used without significantly impacting process latitudes. One reason retaining process latitudes is important is because wafer-to-wafer repeatability is enhanced by wider process latitudes.
After several preliminary preparation steps a wafer is ready to be coated with photoresist. The goal of the coating step is to produce a uniform, adherent, defect-free polymeric film of desired thickness over the entire wafer. Spin coating is by far the most widely used technique to apply such films. This procedure is carried out by dispensing the resist solution onto the wafer surface, and then rapidly spinning the wafer until the resist is essentially dry. In order to maintain reproducible line width in VLSI fabrication applications, resist film uniformity across the wafer (and from wafer to wafer) should be within +-100 angstroms.
The spin coating procedure begins with dispensing the resist solution onto the wafer. The dispensing stage can either be accomplished by flooding the entire wafer with resist solution or by dispensing a smaller volume of resist solution at the center of the wafer. The wafer is then brought to a constant speed spin to distribute the solution evenly over the surface. Once the solution is distributed the wafer is dried by extending the spin. Next the wafer is wet with edge bead removal solution to clear away any excess resist material, and then the wafer is spun dry once again.
During conventional processing the wafers are normally brought as quickly as is practical to the final spin speed. High ramping rates have traditionally yielded better film uniformity than low ramping rates. This is due to the fact that the solvent in the resist evaporates rapidly after the resist has been dispensed onto the wafer. Film thickness depends on the viscosity of the liquid resist solution. As a result, extending the time for the solvent to evaporate by using slower spin-ramp speeds contributes to thickness non-uniformity created by the drying and film setting-up tendencies of the solution. High spin-ramp speed, however, contributes to higher maintenance costs resulting from excessive wear on the motor. These same concerns arise when a solvent with a high evaporation rate is employed in a solvent prewet process application.
Wider process latitudes, such as extended processing time, may be obtained by employing a solvent with a slower evaporation rate. Conventional systems spin a wafer having high evaporation rate solvent for no more than one second. In contrast, a process using a low evaporation rate solvent could spin the wafer up to five seconds. This extended spin period would increase the repeatability across fabrication machines. The trend in conventional processes is to modify the spin-ramp speed, compromising between coverage quality and the cost of processing. They use a spin-ramp speed which gives a sufficiently uniform coating of the wafer surface while reducing motor-wear problems. What is needed is a way to maintain film uniformity at lower ramping rates. This would provide the same (or better) quality of wafer coverage, and would result in reduced processing costs by enhancing repeatability and reducing equipment maintenance.
The photoresist is deposited on each wafer after the wafer is mounted in the process bowl of a track coating unit. One partial solution to controlling the amount of solvent (and other photoresist materials) used is to use a chemical dispensing unit which provides tighter control over the amount of solvent deposited on any one wafer. One such system is described in European Patent 618,504, issued to Hasebe. Hasebe describes a system employing a specialized dispensing head which has a single nozzle for dispensing solvent and a single nozzle for dispensing resist solution. Hasebe controls where the material is dispensed on the wafer by moving the dispense head to different locations relative to the wafer. The system disclosed in Hasebe requires, however, a number of specialized devices, including a moveable dispense head, a pump for the solvent and a temperature adjustment mechanism. This type of system reduces the waste of solvent resulting from over-application, as well as increasing wafer-to-wafer consistency due to the more accurate dispensing of the material. This is only a partial solution, however, because even though a variety of units for dispensing chemicals in this manner are marketed, the units designed for low volumes of low viscosity fluids. In addition, each unit is a specialized system, so when a shop wishes to employ such a method the shop has to retrofit or replace existing equipment. This results in reduced production flow, and overhead costs are significantly increased.
What is needed is a way to improve the dispensing of chemicals on conventional systems. There are companies which manufacture a special purpose pump and nozzle system which can be added to their own track coating equipment manufactured. These pumps can dispense small amounts of fluid accurately and repeatably. Such systems attempt to improve resist throughput and yield by simplifying the liquid supply system and making it more precise. However, in addition to the moveable dispense head and the pump itself, the wafer manufacturer must also purchase a pump controller and special circuitry for communication between the track coating system to that controller. In most manufacturing locations fabrication space is at a premium. Where the location is retrofitting existing equipment, there must be room in the equipment for the pump, the pump controller and other special circuitry. As the pump alone can range anywhere from the size of a shoebox to the size of a computer monitor, it becomes difficult for existing facilities to incorporate the conventional specialized dispense systems.
The fabrication recipe also becomes much more complex as a result of having to direct positioning of the dispense head. One effect of this configuration is that these methods are not easily retrofitted or transferrable to other systems. As a result, the problem of elevated processing costs and restricted application still exist. There remains a need for a chemical dispensing system which can be incorporated into a variety of wafer processing equipment without significantly increasing the costs of wafer processing. In addition, there is a continuing need to use the chemicals more efficiently.